1. Technical Field
The present invention relates to a method for manufacturing an optical element that has a polarizing beam-splitting function. The invention also relates to an optical element.
2. Related Art
Among optical elements having a polarizing beam-splitting function, there is known a polarizing beam-splitting element of a wire grid type. JPA-2006-133275 is an example of related art. The wire grid type polarizing beam-splitting element is manufactured for example through the following steps. A concavo-convex structure, which serves as a diffractive structure, is firstly formed on a light transmissive substrate by using a photolithography method, an etching method or the like. Subsequently an aluminum film that covers a surface of the concavo-convex structure is formed. The aluminum film is then patterned so as to form fine metal lines that have a stripe shaped pattern on the concavo-convex structure by using a photolithography method, an etching method or the like.
According to the above-described method for manufacturing the optical element, in the step where the aluminum film provided on the concavo-convex structure of the substrate is patterned by a photolithography method or an etching method, resist that covers the aluminum film is firstly formed and the resist is subsequently exposed so as to obtain a desired pattern of the fine metal lines.
The diffractive structure requires a depth of at least around 100 nm in the concavo-convex face, which makes it almost impossible to apply a low-viscosity resist that is made for nano-fabrication in a thin and uniform layer onto such concavo-convex face. In order to leave the resist on the convex portion (a part protruded from the substrate side) in sufficient thickness, the concave portion (a part dented toward the substrate side) is inevitably filled with the resist. Thereby the depth of the resist in the concave portion largely differs from that in the convex portion.
In consequence, the amount of light absorption by the resist differs between the concave portion and the convex portion when the exposure is carried out. More specifically, insufficient exposure occurs in the concave portion while excessive exposure occurs in the convex portion, accordingly a width of the resist in a shorter direction becomes large at the concave portion and a width of the resist in a shorter direction becomes small at the convex portion.
As a result, widths of the fine metal lines in the shorter direction are made not uniform since they are fabricated by etching that uses the above-described resist. With such fine metal lines, it is impossible to improve a transmission rate or contrast (an extinction ratio) of the polarizing element in full measure.
Meanwhile, the resist for the fine metal lines can be formed by a nano-imprinting method instead of the photolithography method. However it is difficult to align and imprint a resist pattern that reaches the bottom for an appropriate etching onto the concavo-convex face, and it significantly impairs efficiency in terms of mass production and the like.